[PATCH v6 net-next 08/14] tcp: accecn: retransmit downgraded SYN in AccECN negotiation

chia-yu.chang
2025-11-14T07:14:11Z

Based on AccECN spec (RFC9768), if the sender of an AccECN SYN
(the TCP Client) times out before receiving the SYN/ACK, it SHOULD
attempt to negotiate the use of AccECN at least one more time by
continuing to set all three TCP ECN flags (AE,CWR,ECE) = (1,1,1) on
the first retransmitted SYN (using the usual retransmission time-outs).

If this first retransmission also fails to be acknowledged, in
deployment scenarios where AccECN path traversal might be problematic,
the TCP Client SHOULD send subsequent retransmissions of the SYN with
the three TCP-ECN flags cleared (AE,CWR,ECE) = (0,0,0).

Signed-off-by: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>

---
v5:
- Update commit message and the if condition statement.
---
net/ipv4/tcp_output.c | 15 +++++++++------
1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 479afb714bdf..8039c726d235 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -3571,12 +3571,15 @@ int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
tcp_retrans_try_collapse(sk, skb, avail_wnd);
}

- /* RFC3168, section 6.1.1.1. ECN fallback
- * As AccECN uses the same SYN flags (+ AE), this check covers both
- * cases.
- */
- if ((TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN_ECN) == TCPHDR_SYN_ECN)
- tcp_ecn_clear_syn(sk, skb);
+ if (!tcp_ecn_mode_pending(tp) || icsk->icsk_retransmits > 1) {
+ /* RFC3168, section 6.1.1.1. ECN fallback
+ * As AccECN uses the same SYN flags (+ AE), this check
+ * covers both cases.
+ */
+ if ((TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN_ECN) ==
+ TCPHDR_SYN_ECN)
+ tcp_ecn_clear_syn(sk, skb);
+ }

/* Update global and local TCP statistics. */
segs = tcp_skb_pcount(skb);
--
2.34.1